The invention relates to emergency lights and particularly to using emergency lights together with a super capacitor.
According to law, public buildings must have emergency lighting in the event of a power failure. These systems are defined by standards EN 1838 and EN 50172. In practice, the systems are provided with a battery back-up. Other types of standby electric sources are also used, e.g. hospitals may have a standby electric supply powered by a combustion engine.
Standby lights are most commonly implemented by fluorescent lamps, because they provide a good relationship between service life and luminous efficiency. A problem with fluorescent lamps is the mercury used in them. Today door lights in particular are already realized mainly with LEDs.
Batteries used in emergency lights have also developed greatly. Lead acid batteries and NiCd batteries also contain environmentally hazardous substances. One significant way of storing energy is a capacitor. Today capacitors are almost the only reserve energy sources used for buffer memories in information technology. As a result, the capacitor industry has started to develop ‘super capacitors’. Super capacitors employ double-layer technology and are low-voltage high-capacitance capacitors.
By modern technology it is thus possible to eliminate poisonous substances from standby lights entirely. This can be realized by joining a LED light source and the advantages of a super capacitor.
Super capacitors provide advantages, which make them very attractive for use in emergency lighting. The service life of a capacitor is approximately ten years, which is about twice as much as that of presently used batteries. Capacitors may also be charged up to a desired voltage in a few minutes, which is only a fraction of the time needed for charging batteries.
Correspondingly, LED light sources are very well suited for emergency lights. While in operation, LEDs consume a very small amount of power, wherefore it is sensible to use them in conjunction with capacitors.
Publication US2006/0044801 discloses a principle of using LEDs together with a capacitor. The solution presented in the publication provides a very short-term emergency lighting, because in the circuit presented in the publication the voltage of the capacitor drops quickly.
The voltage of super capacitors is not comparable to the cell voltage of batteries, but a super capacitor may in principle be charged to an arbitrary voltage. However, super capacitors do have a voltage to which it is recommendable to charge them, and voltages higher than the recommended voltage reduce the service life of the capacitor significantly.
Super capacitors must thus be charged carefully especially in conjunction with emergency lights or similar lights subject to official requirements to ensure that the service life of the super capacitor is as long as possible and that the charging of the capacitor is reliable, and hence to allow the emergency light to be used when the normal supply network is down.
If the capacitor is charged in a voltage-controlled manner, which means that the charging apparatus charges the capacitor to a predetermined voltage, it is difficult to check the condition of the connection between the charging apparatus and the capacitor. When the capacitor charged to the desired voltage is detached from the charging apparatus, there occurs no change in the charging current or the output voltage of the charging apparatus. Consequently, damages in the circuitry between the charging apparatus and the capacitor cannot be detected. European standard EN60598-2-22, for example, requires charging circuit indication to be provided. Regular NiCd batteries may be charged using constant current, whereby the performance of the battery circuit may be deduced directly on the basis of the charging current.